Marginalism, how much our items really worth? If you we're stranded in the desert and were given the choice, would you choose a diamond over water? The water has more marginal benefit to you because you need it at that point in time. However, the diamond has more marginal value because you could sell it for lots of money. Reluctantly, the smart move would be to take the water, unless of course you are survivor man, in which case the diamond may seem more worthwhile.